1. Field of the Invention
The present invention relates to an element having a microstructure and a method of manufacturing the same, and more particularly, it relates to an element having a microstructure formed by anodic oxidation and a method of manufacturing the same.
2. Description of the Background Art
In general, a method employing photolithography and etching or a method employing anodic oxidation is known as a method of manufacturing a microstructure of a micro lattice pattern or the like. In recent years, an element having a micro grating groove pattern such as an optical element has been implemented through photolithography and etching.
FIG. 45 is a perspective view showing the concept of a conventional wave plate (polarization element) 100 serving as an element having a microstructure. In the conventional wave plate 100, groove patterns constituting a grating are formed on a glass substrate 101, as shown in FIG. 45. The grating groove patterns are formed by air layers 102 and substrate material layers 103, having a width a, consisting of the same material as the glass substrate 101. The grating groove patterns have a period P not more than the wavelength of light. It is assumed that the refractive indices of the air layers 102 and the substrate material layers 103 (the glass substrate 101) are 1 and n respectively. When light is incident upon the grating groove patterns of the wave plate 100, the wave plate 100 exhibits an effective refractive index corresponding to the mixture of the refractive indices 1 and n of the air layers 102 and the substrate material layers 103.
FIG. 46 is a correlation diagram showing the relation between the effective refractive index and the duty ratio of the conventional wave plate (polarization element) 100 shown in FIG. 45. Referring to FIG. 46, the vertical axis shows the effective refractive index, and the horizontal axis shows the duty ratio (a/P), i.e., the ratio of the width a of the substrate material layers 103 to the period P of the grating. Further, symbol TE denotes light having a direction of polarization parallel to the extensional direction of the grating groove patterns, as shown in FIG. 45. Symbol TM denotes light having a direction of polarization perpendicular to the extensional direction of the grating groove patterns, as shown in FIG. 45.
Referring to FIG. 46, the effective refractive index varies with the duty ratio of the grating groove patterns. In this case, the effective refractive index of the light TE having the direction of polarization parallel to the grating groove patterns differs from that of the light TM having the direction of polarization perpendicular to the grating groove patterns. The characteristic of the effective refractive index varying with the direction of polarization of light is referred to as a birefringence property. Generally known is a polarization-dependent diffraction grating (polarization-dependent diffraction element) capable of presenting no refractive index modulation with respect to light having a prescribed direction of polarization while presenting refractive index modulation only with respect to light having a direction of polarization perpendicular to the prescribed direction of polarization of the said light. The conventional polarization-dependent diffraction grating is now described.
FIG. 47 is a plan view showing grating groove patterns of a conventional polarization-dependent diffraction grating (polarization-dependent diffraction element) having a microstructure. Referring to FIGS. 46 and 47, rectilinear grating groove patterns 100a and rectilinear grating groove patterns 100b extending substantially perpendicularly to the grating groove patterns 100a are alternately formed on a glass substrate 101 in the conventional polarization-dependent diffraction grating. The grating groove patterns 100a and 100b have different duty ratios D1 (=(P−W1)/P) and D2 (=(P−W2)/P) respectively. The grating groove patterns 100a and 100b have the same period P. In other words, the duty ratios D1 and D2 of the grating groove patterns 100a and 100b are adjusted by adjusting the widths W1 and W2 of grooves of the grating groove patterns 100a and 100b respectively.
When light having a direction TE of polarization parallel to the grating groove patterns 100a having the duty ratio D1 is incident, the direction of this light is a direction TM of polarization perpendicular to the grating groove patterns 100b in the grating groove patterns 100b having the duty ratio D2. Therefore, both the effective refractive indices of the grating groove patterns 100a and 100b having the duty ratios D1 and D2 correspond to N5, as shown in FIG. 46. When light having the direction TM of polarization perpendicular to the grating groove patterns 100a having the duty ratio D1 is incident, on the other hand, the direction of this light is the direction TE of polarization parallel to the grating groove patterns 100b in the grating groove patterns 100b having the duty ratio D2. Therefore, the effective refractive indices of the grating groove patterns 100a and 100b having the duty ratios D1 and D2 correspond to N4 and N6 respectively, as shown in FIG. 46. Thus, the effective refractive indices of the grating groove patterns 100a and 100b having the duty ratios D1 and D2 can be equally set to the level N5 with respect to the light having the direction TE of polarization parallel to the grating groove patterns 100a, whereby the grating groove patterns 100a and 100b can be brought into a state (transparent) exhibiting no refractive index modulation only with respect to the light having the direction TE of polarization parallel to the grating groove patterns 100a. 
As a manufacturing process for the rectilinear grating groove patterns of the conventional wave plate 100 shown in FIG. 45 or the rectilinear grating groove patterns 100a and 100b of the conventional polarization-dependent diffraction grating shown in FIG. 47, a method of forming rectilinear grating groove patterns by etching the surface of a glass substrate by photolithography and etching is conceivable, for example.
In the case of forming the rectilinear grating groove patterns of the conventional wave plate 100 shown in FIG. 45 or the rectilinear grating groove patterns 100a and 100b of the conventional polarization-dependent diffraction grating shown in FIG. 47 by photolithography and etching, however, it is difficult to form grating groove patterns having a large depth with a uniform groove width along the depth direction. More specifically, rectilinear grating groove patterns deeply formed by photolithography and etching have trapezoidal sections non-uniform in the depth direction as shown in FIG. 48, and hence duty ratios in upper and lower portions of the grating groove patterns disadvantageously differ from each other. Consequently, it is difficult to form an element having a microstructure of excellent grating groove patterns or the like, and hence it is disadvantageously difficult to obtain an optical element having an excellent birefringence property.
H. Masuda et al., “Appl. Phys. Lett.”, Vol. 71 (19), 10 Nov. 1997, pp. 2770-2772 discloses a process of manufacturing a triangular lattice pattern employing anodic oxidation. The process of manufacturing a triangular lattice pattern disclosed in this literature, capable of forming a triangular lattice pattern having deep and uniform micropores, is proposed as a process of preparing a two-dimensional photonic crystal. More specifically, a valve metal such as aluminum, titanium or tantalum or a semiconductor such as Si or GaAs has such a characteristic that an oxide film having micropores arranged perpendicular to the film surface is formed when an anode is electrified in an acidic electrolyte. In particular, an oxide film of aluminum has such a material characteristic that micropores are easily arranged in the form of a triangular lattice. A triangular lattice pattern having deep and uniform micropores can be formed through this characteristic.
FIGS. 49 to 52 are sectional views for illustrating a conventional process of manufacturing a triangular lattice pattern by anodic oxidation. FIG. 53 is a plan view showing a two-dimensional photonic crystal. The conventional process of manufacturing a triangular lattice pattern by anodic oxidation is now described with reference to FIGS. 49 to 53.
In the conventional process of manufacturing a triangular lattice pattern by anodic oxidation, projecting portions 116a arranged in the form of a triangular lattice are formed on the surface of a press member 116 consisting of a hard material such as SiC, as shown in FIG. 49. Texturing is performed by pressing the press member 116 against the surface of an aluminum material 115. Thus, concave portions 115a arranged in the form of a triangular lattice are formed on the surface of the aluminum material 115, as shown in FIG. 50. Then, the aluminum material 115 formed with the concave portions 115a is oxidized in an electrolyte 119, as shown in FIG. 51. In this case, a cathode 118 is prepared from platinum or the like, and the electrolyte 119 is prepared from an aqueous solution of sulfuric acid, oxalic acid or phosphoric acid. Thus, an aluminum oxide (alumina) film 113 having deep and uniform micropores 113, starting from the concave portions 115a, arranged in the form of a triangular lattice is formed in a self-organized manner, as shown in FIGS. 52 and 53. The micropores 113a can be formed to have a depth of at least 10 μm with respect to submicron diameters.
However, the aforementioned conventional method of manufacturing a triangular lattice pattern by anodic oxidation has been known as a method of forming two-dimensional photonic crystal micropores. In general, therefore, there has been no attempt of forming linear grating groove patterns shown in FIG. 45 or 47 by anodic oxidation.
As hereinabove described, it has been difficult to form a linear grating groove pattern having a large depth with a uniform groove width along the depth direction in general, and hence it has been difficult to form an element having a microstructure of an excellent grating groove pattern or the like.